The invention relates to the field of input/output (I/O) circuitry for an integrated circuit (IC), and in particular to I/O circuitry for an IC that provides an adequate current path between an I/O pad and a source of potential, and guarantees a known state on the I/O pad, during times when the I/O voltage is floating.
Many ICs have I/O interface circuitry and pads that are supplied an I/O voltage, VI/O, while the core logic it interfaces to is supplied with a core voltage, Vcore. When such ICs are used in certain applications, the pad of the IC is required to be capable of sinking a specified amount of current while the I/O supply voltage is floating. Also during these times, it is advantageous to have a known state needed on the pad. Since the pad""s driving and protection circuitry are controlled by the I/O voltage, there is no guaranteed that these circuits will provide an adequate current path between the pad and a source of potential (e.g., ground) when the I/O voltage is floating.
Thus, it would be advantageous to have a means of providing an adequate current path and known state for an IC""s I/O pad during periods when the I/O voltage is floating.
In one aspect of the present invention, an integrated circuit is provided. The integrated circuit has an I/O pad and at least one I/O transistor coupled between the I/O pad and a source of potential. A combinatorial circuit is connected to the I/O transistor""s gate to turn on the transistor during periods that an I/O voltage is likely to be floating. Turning the I/O transistor on pulls the pad to a known state and provides a current path between the pad and the source of potential.
In a particular embodiment, the combinatorial circuit has a first input coupled to an output signal of the integrated circuit and a second input coupled to an enable signal. A particular state of the output signal and the enable signal causes the combinatorial circuit to output a signal that turns the I/O transistor on.
Another aspect of the present invention provides a method of pulling an I/O pad to a known state and providing a current path between the pad and a source of potential during periods when an I/O voltage is likely to be floating. At least one I/O transistor coupled between the I/O pad and a source of potential is provided. Also provided is a combinatorial circuit connected to the I/O transistor and an enable signal such that the combinatorial circuit turns on the transistor in response to the enable signal being in a first state. The enable signal is switched into a first state during periods when an I/O voltage is likely to be floating.